So-called “Darlington pair” amplifiers are used in a wide variety of devices. As the term is used herein, a “Darlington pair” is a compound structure consisting of two transistors (either integrated or separate devices) connected in such a way that the current amplified by the first transistor is amplified further by the second transistor. In general, this configuration gives a much higher current gain than each transistor taken separately and, in the case of integrated devices, can take less space than two individual transistors because they can use a shared collector. Integrated Darlington pairs often come packaged in transistor-like integrated circuit packages.
FIG. 1 illustrates a first embodiment of a basic self-biased Darlington pair amplifier 100. Darlington pair amplifier 100 includes resistors 112, 114, 116 and 118, coupling capacitors 122 and 124, and transistors 132 and 134. Resistors 112 and 114 bias transistors 132 and 134. Resistor 112 also acts as a feedback resistor. Resistor 116 acts as a current limiter as well as for matching purposes. Resistor 118 also acts as a current limiter as well as to improve the linearity of the overall amplifier. The size of transistor 134 is normally larger than the size of transistor 132, and the currents through transistors 132 and 134 are proportional to the size ratio between them.
One problem with Darlington pair amplifier 100 is that across a wide range of temperature, the currents through transistors 132 and 134 change significantly. The voltage across the base-emitter junction (Vbe) of each of the transistors 132 and 134 decreases with increasing temperature, and vice versa. So, assuming that the resistor values do not change with temperature, the voltage across resistor 116 and resistor 118 is higher at high temperatures, due to the lower Vbe of transistors 132 and 134. Hence more current flows through the collectors of transistors 132 and 134 at high temperatures. At lower temperatures, the opposite holds true.
Depending on the process, the total current variation can be up to 50% of the nominal current at room temperature. This is obviously a big problem, as it also affects the RF performance of Darlington pair amplifier 100 over temperature.
FIG. 2 illustrates another embodiment of a basic self-biased Darlington pair amplifier 200 which includes temperature compensation. Darlington pair amplifier 200 is similar to Darlington pair amplifier 100, with the addition of diode-configured transistors 236 and 238, to bias the transistors 132 and 134. Compared to Darlington pair amplifier 100, the value of resistor 114 is reduced and provides an offset voltage required to bias transistors 132 and 134 to the desired current. Because the temperature characteristics of diode-configured transistors 236 and 238 are very similar to the characteristics of transistors 132 and 134, they operate as a temperature compensation circuit. However, because diode-configured transistors 236 and 238 are usually much smaller in size than transistors 132 and 134, the temperature characteristics between the diode-configured transistors 236 and 238 and transistors 132 and 134 do not exactly match. It is also important to place diode-configured transistors 236 and 238 and transistors 132 and 134 as close as possible to each other.
However, Darlington Pair Amplifier 200 has some disadvantages, particularly from the point of view of the RF performance. First, the input impedance match is significantly affected by the presence of diode-configured transistors 236 and 238. Second, diode-configured transistors 236 and 238 increase the overall Noise Figure (NF) of the amplifier.
What is needed, therefore, is a Darlington Pair Amplifier that can operate with reduced current variation versus temperature, while maintaining an acceptable input impedance match and noise figure.